Generally, in a signal processing circuit requiring the recovery of an exact phase, and particularly, when frequency-folded signals are involved, modulated signals and high frequency signals are subjected to analog-to-digital conversion at the receiver, and an exact phase has to be recovered.
For example, according to Japanese Patent Laid-Open Publication No. 2-30292, clocks phase-synchronized with sampling clocks of the transmitting end are synchronized with the phase of the receiving end.
That is, first phase reference signals, which are synchronized with color subcarriers and sampling clocks for predetermined samples of the clocks of the transmitting path, are transmitted after sampling them. Then, at the receiving end, the received signals are phase-compared with second phase reference signals which are generated from a regenerating clock, thereby synchronizing the first sampling clock of the transmitting end with the second sampling clock of the receiving end. As a result, the color burst phase of the television signals regenerated at the receiving end is made to be synchronized with the color burst of the television signals of the transmitting end by carrying out digital-to-analog conversions by means of the sampling clocks which are phase-synchronized with the sampling clocks of the transmitting end.
Meanwhile, in the case where a large amount of images has to be sent to a predetermined band as in the case of HDTV, the muse method (Multiple Sub-Nyquist Sampling Encoding), high quality VTR and the like, the frequency folding technique is widely used. In such a case, a pattern of phase, used to synchronize the phases of signals at the encoding and decoding ends, can be inserted at the encoding end, and, in the case of VTR, it can be applied to a regenerating circuit.
However, when analogue signals are converted to digital signals by sampling them at the receiving end, if the frequency of signals is lower than the sampling frequency, errors occur in small amounts during the sampling, while, if the frequency of signals is higher, then errors occur in large amounts. Therefore, if the sampling point is to be exactly located, a recognition pattern has to be loaded on the signals at the transmitting end, so that a method for recovering an exact phase at the receiving end can be used based on the inserted recognition pattern. In currently used VTRs, the frequency of signals is lower than the sampling frequency, and therefore, errors occur in small amounts, so that there is no major problem. However, if the band is to be expanded based on the frequency folding method, the frequency of signals become high, when compared to the sampling frequency, with the result that the problem of errors becomes serious.
That is, in the case where the frequency is low as shown in FIG. 1A, the sampling point of data is not significantly displaced, so that no major problem should occur. However, if the frequency is high as shown in FIG. 1B, the sampling point of data is significantly displaced, so that errors occur in large amounts, and that it is highly probable that signals will be erroneously recognized as completely different kinds of signals by the receiving end.